Group F, located along the 

 southern lip of the slough consists 

 of red mangrove peat overlying a 

 layer of calcitic mud. This ar- 

 rangement supports the theory of a 

 general transgression of the sea 

 over south and southwestern Florida 

 (Scholl et al. 1969). 



In Florida Bay, Holocene sedi- 

 ments range in thickness from 15 cm 

 (6 in) in the lake areas to 3 to 

 3.7 m (10 to 12 ft) near Cape Sable. 

 Radiocarbon dating indicates that 

 deposition of these largely uncon- 

 solidated, fine grained calcareous 

 muds began about 4,000 years ago 

 (Scholl 1966), during the Flandrian 

 transgression (rise in sea level) 

 that accompanied the melting of the 

 last continental ice sheets. 



Prior to this time, the bedrock 

 of Florida Bay had been invaded by 

 wetland and terrestial vegetation 

 similar to that of the present day 

 Everglades. The surface was also 

 exposed to the same forces of ero- 

 sion and solution from freshwater 

 rains and runoff. As sea level 

 rose, the gradually encroaching 

 marine waters encountered conditions 

 that were similar to those now 

 existing in the southern Everglades. 

 Scholl (1966) believes that bank 

 formation (the keys of Florida Bay) 

 began early in the bay's history, 

 presumably where slack water condi- 

 tions existed due to converging cur- 

 rents. Hoffmeister (1974) believes 

 the keys and shoals provide a rough 

 tracing of former freshwater drain- 

 age patterns that have been greatly 

 altered by current action, especial- 

 ly near the Coral Keys and the open 

 gulf. Sediment trapping and further 

 vegetational stabilization from man- 

 groves and sea grasses as sea level 

 continued to rise resulted in the 

 present day configuration of man- 

 grove keys. Ball et al. (1967) 



demonstrate that the exposed keys 

 and submerged banks may in fact 

 increase in area due to sediments 

 heaped up during hurricanes. 



Figure 27 presents a cross 

 section of cores from Cross Bank in 

 Florida Bay (near Upper Matecumbe 

 Key) . These cores outline the three 

 major strata of the Holocene sedi- 

 mentary record that occur in Florida 

 Bay: (1) the very few or no shells 

 layer (corresponding to the carbon- 

 ate mud of Fleece 1962); (2) the 

 slightly and moderately shelly layer 

 (corresponding to the shelly sand of 

 Fleece 1962); and (3) the peat lay- 

 er. Peat layering is reported from 

 cores beneath keys (Fleece 1962) and 

 shoals (Turney and Perkins 1972), 

 but is not found elsewhere in Flor- 

 ida Bay away from these structures. 



The carbonate mud layer arises 

 primarily from the action of encrus- 

 ting green algae such as Halimeda 

 sp., Udotea sp., and Penicillus sp., 

 which secrete fragile skeletons of 

 tiny aragonite crystals (Stockman 

 et al. 1967). These same authors 

 estimate that the Penicillus popula- 

 tion alone could account for one 

 third of the lime mud production in 

 Florida Bay. Numerous other species 

 of calcareous algae are believed to 

 make up the difference, in addition 

 to some import from the gulf (Hoff- 

 meister 1974). 



Ginsburg (1956) reports that 

 51% of the sediments by weight in 

 Florida Bay have a mean grain size 

 greater than 1/8 mm as opposed to 

 83% in the nearby Florida Key reef 

 tract. Of this relatively smaller 

 proportion, 87% is of molluskan 

 (76%) and foraminiferan (11%) ori- 

 gin. In contrast to these bay sedi- 

 ments, the shelly sands near the 

 keys contain a considerably greater 

 amount of algal and coral remains. 



63 



